Thin film transistor, and organic EL display thereof

ABSTRACT

Thin film transistor, and organic EL display of the same and method for fabricating the same, including a high temperature substrate of metal or ceramic, a semiconductor layer formed in a region of the substrate having a source region and a drain region, a source electrode in contact with the source region in the semiconductor layer for use as a data line, a pixel electrode formed in each of the pixel region in contact with the drain region in the semiconductor layer, an organic EL layer formed on the pixel electrode, a common electrode formed on the organic EL layer, and a transparent protection film on the common electrode.

This application claims the benefit of the Korean Application No.P2001-0033999 filed on Jun. 15, 2001, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin film transistor (TFT), and moreparticularly, to a thin film transistor having a high temperaturesubstrate, and an organic EL display of an active driving type of thesame, and a method for fabricating the same.

2. Background of the Related Art

In general, a semiconductor thin film used in the thin film transistoris formed by crystallizing an amorphous silicon film formed by plasmaCVD or thermal CVD in a furnace, like an electric furnace, for more than12 hours at a temperature higher than 600° C.

For obtaining a high quality semiconductor thin film having an excellentfield effect mobility and a high reliability, it is required that theamorphous silicon film is heat treated for a long time.

Moreover, since it is required that a substrate in a display permits alight to transmit therethrough, the substrate is transparent. Forfabricating the thin film transistor on the transparent substrate, it isrequired that a silicon semiconductor film is formed on the transparentsubstrate. Since glass, which can be used in a low temperature, is usedas the substrate, it is required that the silicon semiconductor filmformed on the substrate is amorphous, which can be formed at a lowtemperature. Then, the amorphous silicon is crystallized for obtaining afast response of the TFT. However, the substrate can be deformed duringa high temperature heat treatment of crystallization of the amorphoussilicon film.

In general, the substrate of the TFT is formed of silica glass, ornon-alkali borosilicate glass. The substrate of silica glass has anexcellent heat resistance, to suit to a high temperature process. Thesilica glass shows no deformation in a heat treatment at approx. 1000°C. However, the silica glass is very expensive in comparison tonon-alkali borosilicate glass, which is increased by a geometricprogression as a size of the substrate is increased. Therefore, thesilica glass is not suitable for a large sized display due to the highcost.

In the meantime, though the non-alkali borosilicate glass is notexpensive compared to the silica glass, the non-alkali borosilicateglass is not suitable for a high temperature process due to a poor heatresistance. That is, the non-alkali borosilicate glass deforms even in aheat treatment at approx. 600° C., which is particularly significant fora large sized substrate with a diagonal length greater than 10 inches.

Accordingly, though the non-alkali borosilicate glass has mostly beenused for displays owing to the low cost, performance of the thin filmtransistor is not good due to low temperature crystallization of thesemiconductor.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a thin filmtransistor, and an organic EL display of the same and a method forfabricating the same that substantially obviates one or more of theproblems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a thin film transistorwhich permits a high temperature process and has a good performance, andan organic EL display of the same and a method for fabricating the same.

Another object of the present invention is to provide a thin filmtransistor which can drop process cost and enhance a productionefficiency, and an organic EL display of the same and a method forfabricating the same.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the thinfilm transistor includes a semiconductor layer, a gate insulating film,a gate electrode formed on a substrate in succession, wherein thesubstrate is a high temperature substrate formed of a metal or ceramic.

The substrate has no deformation at a temperature ranging 500–1500° C.

The metal is one selected from a group of materials including low carbonsteel, high carbon steel, copper, bronze, tungsten, gold, zinc, andtitanium, and an alloy of the materials, and the ceramic is one selectedfrom a group of material including AlN group, Al₂O₃ group, BeO group,SiC group, and MgO group.

The substrate has oxide films on top and bottom surfaces thereof, whenthe substrate is formed of metal.

In another aspect of the present invention, there is provided an organicEL display including a substrate of metal or ceramic, a semiconductorlayer formed in a region of the substrate having a source region and adrain region, a source electrode in contact with the source region inthe semiconductor layer for use as a data line, a pixel electrode formedin each of the pixel region in contact with the drain region in thesemiconductor layer, an organic EL layer formed on the pixel electrode,a common electrode formed on the organic EL layer, and a transparentprotection film on the common electrode.

The organic EL display further includes a black matrix layer formed onthe source electrode to cover the source electrode, for shielding alight.

In further aspect of the present invention, there is provided a methodfor fabricating an organic EL display, including the steps of (a)providing a substrate of metal or ceramic, (b) forming a semiconductorlayer in a region of the substrate, and forming a gate insulating filmon an entire surface of the substrate, inclusive of the semiconductorlayer, (c) forming a gate line on the gate insulating film to pass aregion of the semiconductor layer, and injecting impurity ions with thegate line used as a mask, to form a source region and a drain region,(d) forming a first interlayer insulating film on an entire surfaceinclusive of the gate line, and forming a source electrode to be used asa data line on the first interlayer insulating film in contact with thesource region in the semiconductor layer, (e) forming a secondinterlayer insulating film on an entire surface of the substrate,inclusive of the source electrode, and forming a black matrix layer onthe second interlayer insulating film to cover the source electrode, (f)forming a planarization film on an entire surface inclusive of the blackmatrix layer, and forming a pixel electrode on the planarization film incontact with the drain region in the semiconductor layer, (g) forming anorganic EL layer on an entire surface inclusive of the pixel electrode,forming a common electrode in a region of the organic EL layer, and (h)forming a transparent protection film on an entire surface inclusive ofthe common electrode.

The step (a) includes the steps of polishing, and planarizing a surfaceof the substrate, and cleaning the surface of the substrate.

The method further includes the steps of heat treating the substrate toform oxide films on surfaces of the substrate after the step of cleaningthe surface of the substrate.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention:

In the drawings:

FIGS. 1A–1M illustrate sections showing the steps of a method forfabricating an organic EL display in accordance with a preferredembodiment of the present invention; and

FIGS. 2A–2C illustrate sections showing the steps of a method forfabricating a reflective active matrix LCD in accordance with apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

The present invention suggests employing a substrate suitable for a hightemperature fabrication process in fabrication of a low cost thin filmtransistor having an excellent performance, and fabrication of anorganic EL display by using the same. It is required that the substratesuitable for a high temperature fabrication process of the presentinvention shows no deformation within a temperature range of approx.500–1500° C.

Accordingly, the present invention employs metal, such as low carbonsteel, high carbon steel, copper, bronze, tungsten, gold, zinc,titanium, and the like, or an alloy of the metal as a material of thesubstrate, or ceramic, such as AlN group, Al₂O₃ group, BeO group, SiCgroup, MgO group, or the like as a material of the substrate. Titaniumis the most suitable for fabrication of a large sized display becausefabrication and handling is easy and cost is low.

Therefore, as a preferred embodiment of the present invention, a methodfor fabricating a thin film transistor of titanium substrate, and amethod for fabricating an organic EL display by using the same will beexplained, with reference to the attached drawings. FIGS. 1A–1Millustrate sections showing the steps of a method for fabricating anorganic EL display in accordance with a preferred embodiment of thepresent invention.

A desired size of substrate is formed of titanium Ti, subjected tochemical, and mechanical polishing to have a flatness suitable for adisplay, and cleaned by cleaning liquid for removal of impurities fromthe surface of the substrate.

Then, referring to FIG. 1A, an oxide film 20 is formed on top and bottomsurfaces of the substrate 10, by heat treating the substrate itself atapprox. 500–1000° C. under an oxygen atmosphere. The oxide film isformed for prevention of electric leakage to the substrate, andprotection of the substrate from impurities and chemicals duringfabrication. Therefore, if the substrate is not formed of metal, such asTi, i.e., the substrate is formed of an insulating material, such as AlNgroup, Al₂O₃ group, BeO group, SiC group, MgO group, or the like, nooxide film 20 is required.

Then, referring to FIG. 1B, a buffer layer 30 is formed on the oxidefilm 20, for prevention of infiltration of impurities from the substrate10 to a semiconductor layer formed in a later fabrication process forenhancing a performance of the semiconductor layer. The buffer layer 30is formed of an insulating material, such as silicon oxide SiOx, siliconnitride SiNx, and the like.

Referring to FIG. 1C, a semiconductor layer (polycrystalline siliconlayer) 40 is formed on the buffer layer 30, for use as an active layerof a transistor. The semiconductor layer 40 may be formed by formingpolycrystalline silicon on the substrate directly by means of physicaldeposition, such as CVD like LPCVD, or PECVD, evaporation, orsputtering, or by directing an excimer laser beam to amorphous silicon,or simply by heat treating the amorphous silicon in a furnace.

Referring to FIGS. 1D–1E, a photoresist film 42 is formed and patternedon the semiconductor layer 40, and B or P impurity ions are injectedinto the semiconductor layer 40 by using the photoresist film 42 as amask, to form a lower electrode 41 of a storage capacitor. Then, thephotoresist film 42 is removed, and a gate insulating film 50 is formedon an entire surface. A gate electrode material is deposited on the gateinsulating film 50, and patterned, to form a gate electrode line (e.g.,gate lines) 60 or 60′. Then, impurity ions are injected into thesemiconductor layer 40, and the semiconductor layer 40 is heat treated,for defining source and drain regions of the transistor.

Next, referring to FIG. 1F, after a first interlayer insulating film 70is formed on an entire surface, parts of the first interlayer insulatingfilm 70, and the gate insulating film 50 over the source region of thedevice are removed, to form a first contact hole 80 exposing a part ofthe source region of the semiconductor layer 40.

Referring to FIG. 1G, a source electrode (e.g., data line) 90 to be usedas a data line is formed so as to be in contact with the source regionof the semiconductor layer 40 through the first contact hole 80, and asecond interlayer insulating film 100 is formed on an entire surface.

Then, referring to FIG. 1H, a black matrix layer 110 is formed on thesecond interlayer insulating film 100 to cover the source electrode 90,for shielding a light. The black matrix layer is formed for shieldlights between pixels, and protection of the transistor from the lightfrom an inside of the device.

Referring to 1I, a planarization film 120 is formed on an entire surfaceof the substrate, and parts of the planarization film 120, the first,and second insulating film 70, and 100, and the gate insulating film 50over the drain region is removed to form a second contact hole 130 toexpose a part of the drain region in the semiconductor layer 40. Theplanarization film 120 is formed of BPSG, BCB, SOG, and the like.

Referring to FIG. 1J, a pixel electrode 140 is formed on theplanarization film 120 such that the pixel electrode 140 is in contactwith the drain region in the semiconductor layer 40, thereby fabricatingthe thin film transistor.

After fabrication of the thin film transistor thus, referring to FIG.1K, a material, such as CuPC, and NPD, is deposited on an entiresurface, to form a hole injection layer, and/or a hole transport layer150.

Referring to FIG. 1L, a material, such as Alq3 and dopant, is depositedon the hole injection layer, and/or the hole transport layer 150, toform an organic EL layer 160.

Referring to FIG. 1M, an electron injection layer, and/or an electrontransport layer 170 is formed on the organic EL layer 160, and a commonelectrode 180 is formed thereon. Then, a transparent protection film 190is formed on the common electrode 180, to complete fabrication of theorganic EL display. The protection film 190 is formed of a transparentpolymer group material for emission of a light in a top emission type.

Though a process for fabricating an organic EL display by using atransistor with a high temperature substrate is explained up to now, thethin film transistor of the present invention can be applied to a largesized reflective active matrix LCD, and the like.

FIGS. 2A–2C illustrate sections showing the steps of a method forfabricating a reflective active matrix LCD in accordance with apreferred embodiment of the present invention. As the steps of a processfor fabricating a thin film transistor is explained with reference toFIGS. 1A–1J, detailed explanation of which will be omitted.

In order to fabricate a reflective active matrix LCD, it is requiredthat the pixel electrode 140 in the organic EL display is replaced witha conductive reflective film 140′ of aluminum, or the like.

Then, referring to FIG. 2A, an orientation film 200 is formed on theconductive reflective film 140′.

Referring to FIG. 2B, after a liquid crystal layer 210 is formed on theorientation film 200, an orientation film 220 is formed on the liquidcrystal layer 210.

Referring to FIG. 2C, a color filter 230, a common electrode 240, and atop plate 250 are formed on the orientation film 220 in succession, tocomplete fabrication of the reflective active matrix LCD.

Thus, the thin film transistor of the present invention is applicable todifferent displays to which a high temperature process is applicable.

As has been explained, the thin film transistor, and the organic ELdisplay of the same and the method for fabricating the same of thepresent invention have the following advantages.

First, the use of a metal plate or a ceramic substrate, a substrate foruse in a high temperature, permits to fabricate a thin film transistorof an excellent performance, that in turn permits to fabricate a displayof an excellent performance.

Second, as the present semiconductor process is used as it is,fabrication is easy.

Third, as the substrate is formed of metal or ceramic, handling is easy,and weight, thickness, and cost of production of the product can bereduced, a production efficiency can be improved.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the thin film transistor,and the organic EL display of the same and the method for fabricatingthe same of the present invention without departing from the spirit orscope of the invention. Thus, it is intended that the present inventioncover the modifications and variations of this invention provided theycome within the scope of the appended claims and their equivalents.

1. An organic EL display using a thin film transistor having a substrateof ceramic and a matrix of pixel regions, the organic EL displaycomprising: the substrate of ceramic; a semiconductor layer formed on aregion of the substrate having a source region and a drain region; asource electrode in contact with the source region in the semiconductorlayer for use as a data line; a pixel electrode formed in each of thepixel region in contact with the drain region in the semiconductorlayer; an insulating layer between the substrate and the semiconductorlayer, the semiconductor layer and the source electrode, or the sourceelectrode and the pixel electrode; an organic EL layer formed on thepixel electrode; a common electrode formed on the organic EL layer; anda transparent protection film formed of a polymer group on the commonelectrode, wherein the ceramic consists essentially of MgO group, andthe substrate and the thin film transistor are used in a display device.2. An organic EL display as claimed in claim 1, wherein the substrate ofceramic has a diagonal length of at least 10 inches.